Field of the Invention
The present invention relates to a heating device that heats a recording material formed with an unfixed image to fix the image to the recording material, and an image forming apparatus having the heating device.
Description of the Related Art
In an electrophotographic copying machine or printer, there is used a heating device that thermally fixes an unfixed toner image formed on a recording material (such as a transfer sheet or a photosensitive sheet) to the recording material. For example, an on-demand film-heating-type device has been known (see, for example, Japanese Laid-open Patent Publication No. 2002-296955).
In the film-heating-type heating device, a ceramic heater or other heating member is used. The ceramic heater includes a ceramic base plate (e.g. of alumina or aluminum nitride) which is electrically resistant, heat-resistant, and excellent in thermal conductivity. The ceramic heater further includes heating resistors (e.g. of silver palladium) which are pattern-shaped on the base plate e.g. by printing or baking and which generate heat when supplied with electric power, power supply electrodes which are pattern-shaped e.g. by printing or baking, and a conductor pattern which is low in resistance and which connects the power supply electrodes and the heating resistors to one another. The ceramic heater, which is configured as described above to generate heat when the heating resistors are supplied with electric power through the power supply electrodes and the conductor pattern, is low in heat capacity as a whole and therefore high in temperature rise speed.
When recording materials that are small in size in a longitudinal direction of the heating member (i.e., small in widthwise size of recording materials) are continuously thermally fixed in the heating device whose heating member is formed by the ceramic heater which is small in heat capacity, heating member temperature tends to more easily rise at longitudinally opposite end portions of the heating member where recording materials do not pass than at a longitudinal central portion thereof where recording materials pass. In that case, due to the presence of a temperature difference in the longitudinal direction of the heating member, gloss unevenness tends to occur in fixed images. To avoid this, print speed must be decreased or temperature difference in the longitudinal direction of the heating member must be reduced.
Thus, there have been proposed heating devices each having a heating member provided with a plurality of heating resistors and a plurality of temperature detecting elements for detecting temperatures of the heating member at plural longitudinal positions and each configured to control power supply to the heating resistors based on the detected temperatures, thereby preventing gloss unevenness from occurring in fixed images due to a temperature difference in the longitudinal direction of the heating member.
As such a heating device, there is for example a fixing device that has a heating member formed by heating resistors disposed and configured to generate a large amount of heat at parts of the heating member where recording materials of any size pass and by heating resistors disposed and configured to generate a large amount of heat at parts of the heating member where only recording materials which are large in size pass (see, Japanese Laid-open Patent Publication No. 10-177319).
This fixing device controls power supply to the heating resistors based on a temperature detection result, thereby reducing a temperature rise at parts of the heating member where recording materials do not pass and thereby controlling temperatures at parts of the heating member where recording materials pass to predetermined temperatures. However, a conductor pattern (hereinafter, referred to as the electrically conductive parts or the conductive parts) that connects the heating resistors and the power supply electrodes sometimes generates heat, which can cause a problem.
To improve the reliability of connection between component parts formed on the base plate of the ceramic heater and the power supply electrodes, a relative large area is provided for installation of the power supply electrodes. Even in a small-sized heating device having a base plate which is compact in size, a large area is ensured to install the power supply electrodes. Thus, a space occupied by the conductive parts that connect the heating resistors and the power supply electrodes is naturally constrained in the heating device where a large area is provided for installation of the power supply electrodes.
It should be noted that the electrically conductive parts each have a resistance value although it is small. More specifically, the resistance value of each conductive part is inversely proportional to a cross-sectional area of the conductive part, and hence increases with decrease of the cross-sectional area of the conductive part. When electric power is supplied to the heating resistors, there occur power losses each represented by the product of current value and resistance value of the conductive part, and heat is generated.
In a heating device configured to control power supply to a plurality of heating resistors, power supply electrodes are provided for respective ones of the heating resistors, and therefore electrically conductive parts are installed in a more restrained space. In particular, a space for conductive parts, which are connected to power supply electrodes disposed remoter from the heating resistors, is more restrained. This requires the electrically conductive parts to be thinned, and much heat tends to generate.
When heat is generated by the electrically conductive parts that connect the heating resistors and the power supply electrodes, temperatures of the power supply electrodes rise and a temperature of a power supply connector connected to the power supply electrodes also rises. As a result, metallic parts such as copper alloy parts of the connector cannot ensure contact pressures that depend on thermal stress characteristics thereof, and the reliability of the connector can be impaired. On the other hand, in a case where metallic parts of the connector are formed by e.g. copper-titanium alloys to ensure the contact pressures, the reliability of the connector can be ensured, but a problem of increased costs is caused.